Antenna



March 1954 L. c. VAN ATTA ANTENNA 2 Sheets-Sheet 1 Filed Sept. 19, 1945 FlG.l

E Wmm HMUU V- R H U MP5 INVENTOR. LESTER C. VAN ATTA ATTORNEY March 1954 L. c. VAN ATTA 2,671,855

ANTENNA Filed Sept. 19, 1945 2 Sheets-Sheet 2 A FIG.2

FIG.3

X E QG INVENTOR.

LESTER c. VAN ATTA BY I ATTORNEY.

Patented Mar. 9, 1954 ANTENNA Lester G. Van Atta, Winchester, Mass assignor, by mesne assignments, to the United States of America as represented by the Secretary of War A plication September 19, 1945, Serial No. 617,318

12 Claims.

This invention relates to the provision of antenna systems for transmission or reception of high-frequency radio waves and particularly to antenna systems for operation at frequencies in the so-called microwave range, corresponding to Wavelengths of the order of centimeters, which antenna systems are adapted to provide desirable energy transfer characteristics over a range of frequencies. More particularly the invention relates to arrangements for reducing the frequency sensitivity of such antenna systems.

In microwave communication, such as in apparatus for locating and detecting objects by the radio echo methods, it is often difficult to impose strict tolerances on the frequency of transmission, particularly the frequency of operation of the transmitter tube, which tube is commonly of the magnetron type. It is therefore important that various portions of the radio system, and particularly the antenna system (since it is usually at a considerable electrical distance from the transmitter tube), should have broadband characteristics in order that a change of transmitter tubes, and consequently a slight change of operating frequency, can be made without free space, either directly or with the help of a 1? system of reflectors, or to transfer energy vice v rsa n g n r e e y ransf r i n t perfect, which is to say that there are some losses and, more important, that some energy is re.-

flected, in the case of transmission, back into the transmission line.

It has been known for some time that the reflection of energy back into the transmission line may be kept to a very low level for a given frequency of operation. For this purpose various types of matching transformers are used. In general, such devices are effective only in the immediate neighborhood of the chosen operat ing frequency, which is to say that they are relatively frequencyasensitive. which may be a dipole, will also have some frequency sensitivity and a further source of frequency sensitivity which is particularly impor tant is the dependence of impedance match upon the electrical distance between the effective center of radiation and the vertex of the parabolic reflector as described in my copending patent application Serial No. 507,585, filed .Qctober 25, 1943, issued November .1, 1949, as Patent No. 2,486,620.

The term The antenna itself,

My work has led me to believe that the impedance variation with changes in frequency (which is the same thing as the impedance variation with changes in the distance between the vertex of a parabolic reflector and the effective center of radiation when such distance is rneas? ured in wavelengths) arises from the fact that some of the oscillatory energy reflected by the beam-forming reflector is intercepted by the radiating antenna and is thus reflected back into the feed line. Such reflection sets up standing waves in the feed line, and although such standing waves may be cancelled out for a particular design frequency by the provision in the system of a suitable matching transformer at the antenna end of the feed line, such an arrangement will prevent a substantial mismatch only for an extremely narrow range of frequency in the neighborhood of the design frequency of the matching transformer. I have found that the frequency sensitivity of an antenna system in.- cluding a beam-forming reflector situated at some distance from its driving radiator may be greatly improved by locating a matching reflector in the neighborhood of the beam-forming reflector in such a manner that the reflection of energy from the remainder of the reflector is cancelled by an equal and opposite reflection into the feed line by the maching reflector.

It is an object of this invention to provide means for reducing the frequency-sensitivity of microwave antenna systems. It is a further object of this invention to provide means for reducing the internal reflections arising from the antenna system reducing such reflections in a manner involving low frequency-sensitivity. Further objects of the invention will appear from the reading of this specification.

The invention may be explained and described in connection with the annexed drawing, in which:

Fig. 1 is a perspective view, partly broken away, of one form of antenna system adopted for carrying out the present invention;

Fig. 2 shows diagrammatically in side elevation another form of antenna system in accordance with the invention.

Fig. 3 is a partly diagrammatical side elevation of a modification of the antenna system of Fig. 3; and

Fig. 4 shows still another form of antenna system in accordance with the present invention.

The type of beam-forming reflector such as is usually employed with a steerable directive radio object-locating system of very high frequency is a metal parabolic reflector geometrically similar to the parabolic reflector used in searchlights, reflecting telescopes and other optical devices. Such a reflector is commonly illuminated by a radiator or radiating system the effective center of radiation of which is located at or very close to the focus of the parabolic reflector. Usually the edges of the parabolic reflector define a plane perpendicular to the axis of the reflector, but sometimes, when a slightly fanned beam is desired a modified parabolic reflector is used for example one which is cut away on opposite sides of the axis of the reflector, as by two parallel planes equidistant from and parallel to the said axis in the manner illustrated in the case of the reflector appearingin Fig. 4. Other types of reflectors may also be used, for instance parabolic cylinders and pillbox typereflectors and other specially shaped reflectors for obtaining different radiation patterns. Because the beam-forming reflector is usually a parabolic reflector as above described, the in vention is explained and illustrated with reference to antenna systems employing parabolic reflectors and more particularly a reflector formed as a paraboloid of revolution.

In order that a matching impedance, introduced in a transmission line in order to match the transmission line to a load having an impedance other than the characteristic impedance of the transmission line, may have the lowest possible frequency sensitivity, it is desirable that the matching impedance should be located as closely as possible to the load in question.

Thus in order that a matching adjustment may provide reasonably satisfactory matching of line to load for as wide a band of frequencies as possible, it is desirable that the discontinuity in the line which produces the reflections which are designed to cancel the reflection effect of the load should be as close as possible to the load. This principle is applicable to all types of coupling arrangements between systems of travelling waves and systems of standing waves. It is applicable generally to guided waves, both in transmission lines of the usual parallel conductor or coaxial conductor types and also in wave guides in the form of hollow conducting pipes. I have found it applicable also to an enna systems in which a non-resonant reflector is used together with an antenna or antenna array which illuminates such reflector for the formation of a desired directional characteristic. In general, the application of this principle in accordance with the present invention is useful wherever a reflector is illuminated by an antenna element or an antenna array located at a considerable distance in terms of wavelengths, such as one-half wavelength or more from the nearest part of said reflector. It is particularly useful where the said distance is relatively great such as two or more wavelengths, the reduction of frequency sensitivity in such cases being very great.

Fig. 1 shows one way of applying the present invention to a typical form of antenna system.

The antenna system includes the parabolic reflector Ill, a broken away portion of which is shown, the feed or transmission line II, which may be of the hollow wave guide or coaxialconductor type and passes through the vertex of the parabolic reflector if), as shown at E2. A dipole antenna I3 is electrically connected to and mounted on the feed line It, and auxiliary reflector or parasitic element I4 which may be a metal disk is supported on a forward extension of feed line II at a distance of slightly more than a quater wave length in front of the dipole antenna I3. The antenna system may also include sleeve type resonator I5 open at its forward end and closed at its rear end and adapted to prevent undesired coupling between the inside of the transmission line H and the outside cf the outer conductor thereof and thus effectively to balance the excitation of the dipole I3 by the transmission line I I In order to reduce frequency-sensitivity of the antenna system in accordance with this invention, a metallic structure extending radially away from the outside of the transmission line II, is mounted on the outside of the outer conductor of the transmission line I i at a distance from the parabolic reflector I0 determined according to electrical measurements.

Such structure may be curved or flat metallic sheet but in the present case is shown as preferably being an annular metal disk I8. This metallic disk, which may be referred to as a matching reflector or matching disk, is preferably made of such size that the amount of re flection into the transmission or feed line II caused by the disk It will compensate as closely as possible for the reflection into the feed line from the remainder of parabolic reflector I0. In general the metallic disk It is made sufliciently large to provide reflection of the desired portion of energy yet is made sufficiently small relative to the size of reflector I8 in order to prevent it from interfering with the directive properties of the antenna system by blocking off portions of the pattern (aperture) of the parabolic reflector It not already blocked off by the auxiliary reflector [4. From the point of view of obtaining a satisfactory radiation pattern, the size of disk I6 is preferably made as small as possible depending on the size and focal length of reflector Iii. However, satisfactory results can be obtained with a disk I6 with an outer diameter generally smaller than the auxiliary reflector disk I l. The auxiliary reflector disk I4 in accordance with common practice is usually about eight-tenths of the free-space wavelength in diameter.

The disk I6 need not be circular in shape. For instance, it may be elliptical, being longer in the direction parallel to the orientation of the dipole I3. Various other shapes and types of matching reflectors may also be utilized and instead of a flat shape, the disk I6 may be given the form of a curved surface such as a paraboloid confocal with the parabolic reflector I0, as described hereinafter with reference to Fig. 3. The important thing apparently is that the disk I6 should project away from the transmission line II, at least a part of it extending away in a direction approximately parallel to the direction of the dipole I3. It is also desirable that the metallic structure It should be to a considerable degree concentrated in a plane I6 perpendicular to the axis of the feed line I I. The structure I6 may be provided with a suitable mounting flange, as shown in dotted lines at IT, and may be mounted in a suitable fashion on the transmission line I I, by soldering, by a screw-thread type of fitting, or otherwise.

In order to determine the proper location of the structure l6, one or more such structures (preferably several of different sizes) may be mounted on the feed line I I in turn and tested in different positions on the feed line at several frequencies within the rang over which good energy-transfer characteristics are desired. The tests corresponding to different positions of the structure I6 will show some difference between them in the energy-transfer characteristic with respect to frequency and, by observing the resalts of the tests, the location corresponding to the minimum frequency-sensitivity can, be estimated and then checked by a further test. Running tests of thm; type with two or more different sizes of this type of structure in turn will indicate which size, is they best for minimum frequency-sensitivity and, if none of the particular disks tested gives sufficient reduction of the frequency-sensitivity for a particular purpose, the test may indicate in which sense the dimensions of the. disk should be modified in order to obtain better reduction of frequency-sensitivity.

Since the inherent frequency-sensitivity of antenna systems diifers fairly considerably among the various types, it. is likely that for some types of antenna systems the amount of improvement in lowered frequency-sensitivity obtainable in accordance with this invention will not justify the addition to the system of a further structure such as the structure It. On the other hand, in a number of instances of common types of antenna systems, appreciable improvement has been obtained by the above described procedure, and it has been found that once the proper size and location for the structure I6 has been determined for a particular model of antenna systems, the dimensions and spacing thus obtained will hold to provide substantially similar results for other samples of the same model. The chief electrical dimension appears to concern the distance between the structure It and the vertex of reflector l0. It is believed that once a suitable structur l6 has been designed for a given antenna and feed line assembly, including an auxiliary reflector or parasitic element such as the reflector l4, and a suitable location for the structure 16 with respect to the reflector It, such an antenna may be expected to operat with desirable energy transfer characteristics over the intended range of frequencies, provided, however, that equivalent spatial relations of the effective center of radiation and the half-Wave "points are maintained as explained in my aforementioned copending patent application, Serial No. 507,585, which issued November 1, 1949, as Patent No. 2,486,620.

This invention may be considered from another point of view. Thus, it has been noted that the energy returning from the outer portions of a parabolic reflector generally lags that returning from the center portion and this lag may be of the order of a quarter of the wavelength of the radiant energy. This is generally indicated Fig. 2 which shows diagrammatically, a paraboloidal reflector Zii illuminated in a conventional manner by a radiating element 2l. The outer annular portion of reflector 2! may for the present description be shown as an upper and lower portion between the points designated by A-B and CD while the center portion is indicated by 13-0. Thus it may be assumed that the energy reflected from portion B-C leads by a quarter wavelength that reflected from portions A-B and CD. It has been found that that portion of the energy thus reflected by the center portion B-C- is reflected back to the radiating element and causes an effective mismatch at the radiating element which sets up undesirable standing waves in the transmission line connecting the radiating element to the transmitting or receiving apparatu indicated hereinabove.

As will be understood this invention contemplates the nullification of the undesired energy returned by the e tire reflector by u ing the reflection from a portion of the reflector to cancel the reflection from the remainder. To achieve this. a portion such as center portion B-C must be chosen of such diameter that the two reflections will be equal in amplitude and must b moved forward or backward along he. axis so that the two reflections will be opposit in phase. The same effect may be produced by placing a relatively small reflector shaped as a paraboloid of revolution of substantially the same con,- formation as portion B-C and of the proper dimensions along the axis of the reflector at an optimum distance from the reflector as more fully described hereinbefore. In general this small reflector 22, Fig. 2 (which may be designated a matching reflector) may be placed approximately one eighth of the wavelength 8 in front of the main parabolic reflector 20. It will now be understood that the energy reflected back to the radiating element 2| by center portion BC will lead the energy returned by the outer portions AB and CD by which is effective to destroy the interference of mismatched power in the transmission line. In addition to aifording a, matched impedance at the radiating element and in the line, the matching reflector 22 is considerably more broad-band than the usual type of tuning stub type of matching device. This is due to the fact that the distance between th main reflector 20 and the matching reflector 22 is usually very short (less than a wavelength) and. is therefore not frequency critical.

It has been found that with the antenna of Fig. 2 there is some increase in side lobes and a loss of antenna, gain as a result of the addition of the matching reflector 22. However, I have found that these disadvantages may be reduced to a certain degree by moving the matching reflector 22 nearer the radiating element 2|, such as in the position indicated by dotted lines in Fig. 2. The dotted line reflector 22' is placed at a distance d of an integral number of half wavelengths from the position of the solid line matching reflector 22 that is approximately from reflector 20 (n being an integer). It is apparent that the ener y returned by m hing reflector 22 will be out of phase with that from reflector 20. It is therefore important that only a small portion of the energy reflected by reflector 22 reaches the main radiation pattern since such outof-phase energy reduces the main lobe and. increases side lobes causing loss of antenna ain. It will thus be seen that the ma n advantage in moving the matching reflector to the dotted line position is that its proximity to the radiating element or feed 25 causes a greater percentage of the energy reflected thereby to return to the radiating element with a smaller amount going out into the radiation pattern. Another reason for the increase in antenna gain obtained with the reflector 22' in the dotted line forward position is that when so placed. reflector 22 may be made smaller in size thus cutting ofi a smaller part of the energy from the main reflector 20 and affording a greater useful and ef- .fective surface of the main reflector 20.

However, While such advantages may prove useful it has been determined that an antenna having a matching reflector 22' in the dotted line position is not very broad-banded. This is due to the ratio m/p (m and being the distances from radiating element 21 to reflector 22 and to the vertex of reflector respectively) which is the measure of change in the relative phase of energy returned to the radiating element 2| which change in frequency is not sufficiently close to 1. Thus for broad-banding it is preferable that the matching reflector 22 be positioned as close as possible to the vertex region of reflector 25 as indicated hereinbefore with reference to Fig. 1.

According to a modification of the arrangement of Fig. 2, it has been found possible to increase the effective m/p ratio and to achieve more desirable broad banding characteristics. This may be accomplished by placing a plurality of matching reflectors along the axis of reflector 20' as indicated in Fig. 3. Thus several reflectors preferably paraboloidal in shape such as the three reflectors 2 25 and 26 are arranged coaxially along the reflector axis. Each of the matching reflectors 23, 25 and 23 are preferably spaced an integral number of half wavelengths apart with the reflector 25 nearest reflector 25' preferably being from reflector 20. It is also preferable that the size of the matching reflectors decrease in size as they are placed a greater distance from reflectorZO.

Fig. 4 is a perspective view of still another form of antenna system according to the present invention. The apparatus there shown is intended to produce a horizontally polarized beam which is sharp in the horizontal plane but relatively broad in the vertical plane and is therefore adapted for use on shipboard, in which service relatively large variations in elevation are imposed upon the apparatus by the motion of the ship while the variations in azimuth are relatively smaller. Apparatus of this type is also useful for certain types of general aircraft warning service where it is desired to ascertain range and bearing of aircraft more precisely than other factors. In order to produce the desired directional characteristic the principal reflector of the antenna system, shown at 38, is provided in the form of a so-called cut paraboloid which may be described as a strip of an ordinary parabolic reflector out along the intersection of the parabolic reflector with two planes parallel to the geometrical axis of the paraboloid and parallel to each other.

The long dimension of the parabolic reflector 30 is arranged in a generally horizontal direction and the reflector 30 is pivoted about a vertical axis upon a yoke 3! forming part of a steerable mount the rest of which is not shown for convenience of illustration but which corresponds in its general features to a searchlight mount. The reflector 39 may also pivot about a horizontal axis on yoke 35, the elevation angle being controlled by a motor 32 which may form a part of a servo mechanism. The parabolic reflector 30 is illuminated by a wave guide pipe 33 which is mounted in fixed relation to the yoke 3| and is carried with the said yoke when the latter is turned in azimuth. The pipe 33 is vertically oriented but its upper end is closed off at an angle of about '45", such termination forming a reflector 34 adapted to reflect oscillatory energy through an opening 35 in one of the narrower sides of the pipe 33 towards the parabolic reflector 30. However pipe 33 may also be terminated in a horn type radiating element of any suitable design.

The pipe 33 in the form shown is rectangular in cross-section and is adapted to be excited in the TE0,1 mode at the frequencies of operation. Since the opening 35 is in the narrow wall of the pipe 33, the radiation issuing out of the said opening in said case is horizontally polarized, the electric vector in the TE0,1 mode being parallel to the shorter cross-sectional dimension of the pipe. Conducting curtains 36 which constitute a matching diaphragm constricts the pipe in the magnetic plane (in the direction of the broader dimension) and are of such size, configuration and location as is adapted to cancel reflections occurring from the discontinuities in the neighborhood of the orifice 35. At its lower end the pipe 33 is coupled to a coaxial conductor transmission line 31 which serves to connect the pipe 33 and the antenna system with the transmitting and receiving apparatus, which is not shown in the drawings.

The method of illuminating the parabolic reflector 39 by means of the pipe 33 as shown in Fig. 4 is considerably less sensitive to changes in frequency even apart from the means hereinafter described relating to the present invention, than other systems heretofore used for obtaining a steerable horizontally polarized beam which is sharp in the horizontal plane and somewhat broader in the vertical plane. I'he frequency sensitivity of the antenna system is further reduced in accordance with the principles of this invention by the provision of an auxiliary reflector 35. which is adapted to cancel the effects of undesired reflection of energy from the parabolic reflector 30 back into the pipe 33. If the parabolic reflector 33 were maintained in fixed relation with the wave guide pipe 33 the auxiliary reflector 33 would preferably be provided in the form of a centrally located conducting disk at an optimum distance as determined by experiment or calculation in front of the vertex of the parabolic reflector 4-5. Such distance is preferably about one-quarter wave length. In order to obtain a good radiation pattern in the horizcntal plane it is desirable to avoid placing or extending the matching reflector in the horizontal plane. Therefore, matching reflector 4B is kept as small as possible in the horizontal plane and is enlarged in the vertical plane to give the required area for producing the desired results. The reflector 5B is provided in the form of a strip, oriented vertically with respect to the pipe 33 or the horizontal axis of reflector 38. The reflector 33 may be supported in any of a number of ways upon the parabolic reflector 312. I prefer to support it upon a sheet metal web 41 oriented in the magnetic plane. a

In order to reduce the cancellation by the reflector 45 of waves that would otherwise be radiated into space off the parabolic reflector 30 the apparatus of Fig. 4 might be modified by substituting for the reflector 53 supported upon the parabolic reflector 3G, a reflector supported upon a structure extending from the pipe 33 adapted to maintain such auxiliary reflector at a distance of substantially one-quarter wave length from the surface of the parabolic reflector 30 but adapted to maintain its position with respect to the pipe 33 irrespective of themovernent of the parabolic reflector 30. Another possible arrangement to mitigate effect of the auxiliary reflector 48 upon the radiation characteristics of the antenna system, other than the reduction of frequency sensitivity is the mounting of the wave guide pipe 33 upon a frame adapted to swing in fixed relation with the parabolic reflector 39 instead of mounting the pipe 33 upon the yoke 3! as shown in Fig. 4. In such case the reflector :36 is preferably made in the form of a circular or parabolic disk such as is shown in Figs. 1, 2 and 3.

The reduction of the frequency sensitivity possible by practice of the present invention in the case of antenna systems where the ratio of the focal length to the wave length is greater than that illustrated described in connection with Fig. i. is all the more significant because of the greater frequency sensitivity of such antenna e ms as heretofore constructed. As an example or" what can be accomplished according to the present invention; antenna which originally a band width oi only 3% and in which the SWR tending wave ratio) was less than 1.4, was inc-re sad to by the use of a match ing reflector in front of main reflector as hereinoefore described.

While there has been described what is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. In combination, a wave guide connected at one end to a transmitter and having a radiating element at its other end, and means for preventing standing waves in said wave guide comprising a pair of reflectors of different size and facing said radiating element, the difference in size of said reflectors being dependent upon the integrated amplitude, and the diiference in said distances being dependent on the integrated phase, of the energies propagated from said radiating element to said reflectors and returned to said radiating element.

2. In combination, a large paraboloidal reflector having an axis and a vertex, a wave guide having at one end a radiating element, said radiating element extending perpendicular to and having its midpoint on said axis, a small reflector located in front of said large reflector and. having its midpoint on said axis, the spacings between said radiating element and said reflectors being diflerent, and a transmitter connected to the other end of said waveguide.

3. The combination in accordance with claim 2, wherein said waveguide passes through the vertex of said large reflector and is located approximately coaxially therewith, wherein said radiating element is a dipole antenna electrically connected to and supported at said one end of said wave guide, and wherein said small reflector comprises a metallic structure mounted on the outside of said waveguide and located between said dipole antenna and said large parabolic reflector, said structure including a metallic surface approximately perpendicular to said wave guide having a substantial extent at least in a direction parallel to said dipole antenna; and said combination further including an auxiliary reflector on the opposite side of said dipole antenna from said large parabolic reflector.

4. The combination according to claim 3, in which the said metallic structure is substantially in the form of a sheet metal plate.

5. The combination according to claim 3 in 10 which said metallic structure is substantially in the form of a circular disk.

6. The combination according to claim 3, in which said metallic structure has substantially a parabolic surface.

7. A broad band antenna comprising a main parabolic reflector, a radiating element disposed approximately at the focus of said parabolic reflector, a plurality of impedance matching reflectors spaced one-half wavelength apart along the axis of said parabolic reflector between said radiating element and said parabolic reflector, the matching reflector proximate said main reflector being spaced therefrom approximately one-eighth of the operating wavelength.

8. The antenna according to claim 7, wherein said impedance matching reflectors comprise paraboloids of graduated aperture, the larger of said paraboloids being proximate said main reflector.

9. In combination, a dielectric guide connected at one end to a transmitter and having an antenna aperture at its other end, and means for preventing standing waves in said guide comprising a pair of reflectors of different size positioned at diflerent distances from and facing said aperture the difference in size of said reflectors being dependent upon the integrated amplitude, and the difierence in said distances being dependent upon the integrated phase, of the energies propagated from said aperture to said reflectors and returned to said aperture.

10. In combination, a large paraboloidal reflector having an axis and a vertex, a dielectric guide having at one end an antenna aperture, said aperture extending perpendicular to and having its mid-point on said axis, a small circular or disk reflector attached to the front of said large reflector and having its mid-point on said axis, the spacings between said aperture and said reflectors being diflerent, and a transmitter connected to the other end of said dielectric guide.

11. An antenna system comprising a radiating element, a large reflector illuminated by electromagnetic energy from said radiating element for forming a directional beam of reflected electromagnetic energy, a portion or said reflected energy impinging on said radiating element and inducing a first voltage therein, a small matching reflector located intermediate said radiating element and said large reflector and illuminated by energy from said radiating element, from the geometrical center of said matching reflector to said radiating element being such that reflected energy from said matching reflector impinging on said radiating element induces a second voltage therein which is substantially out of phase with said voltage.

12. An antenna system as set forth in claim 11, wherein the size of said matching reflector relative to the size of said large reflector is such that the amplitude of said second voltage is substantially equal to the amplitude of said first voltage.

LESTER C. VAN ATTA.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,118,419 Scharlau May 24, 1938 2,175,254 Carter Oct. 10, 1939 2,370,053 Lindenblad Feb. 20, 1945 2,407,057 Carter Sept. 3, 1946 2,489,865 Cutler Nov. 29, 1949 

